PTERIDOPHYTA (Vascular Cryptogams).
The Pteridophytes include plants which vary in size from a few
millimetres[482] to several metres in height. The spore on germination
gives rise to a small thalloid structure, the _prothallium_, on which
the sexual organs are developed; this is the _gametophyte_ or sexual
generation. The sexual organs have the form of typical archegonia
and antheridia. From the fertilised egg-cell there is developed the
Pteridophyte plant or _sporophyte_, which bears the spores. This
asexual generation shows a well-marked external differentiation into
stem and leaves, and bears true roots. Internally the tissues exhibit
a high degree of differentiation into distinct tissue-systems. True
vascular bundles occur, which may or may not be capable of secondary
thickening by means of a _cambium_, _i.e._ a definitely localised zone
of meristematic tissue. The sporangia are borne either on the ordinary
foliage leaves or on special spore-bearing leaves called _sporophylls_,
which differ in a greater or less degree from the sterile leaves.
The majority of the best known and most important Palaeozoic genera
are either true Vascular Cryptogams, or possess certain of the
pteridophytic characteristics combined with those of higher plants.
It is not merely the commoner and more familiar recent genera with
which the student of extinct types must be acquainted, but it is
extremely important that he should make himself familiar with the
rarer, less known and more isolated recent forms, which often throw
most light on the affinities of the older representatives of the group.
It is often the case, the more isolated living plants are, the more
likely are they to afford valuable assistance in the interpretation
of genera representing a class, which reached its maximum development
in the earlier periods of the earth’s history. The importance of
paying special attention to such recent plants as may be looked upon
as survivals of a class now tending towards extinction, will be more
thoroughly realised after the extinct vascular cryptogams have been
dealt with.
A comparison of the Pteridophyta and Bryophyta brings out certain
points of divergence. In the first place, the sporophyte assumes in
the former class a much more prominent rôle, and the gametophyte has
suffered very considerable reduction. The gametophyte, _i.e._ the
structure which is formed on the germination of the asexually-produced
spore, is usually short-lived, small, and more or less dependent on
the sporophyte for its nutrition. In a few cases only is it capable
of providing itself with the essential elements of food. On the
other hand, the sporophyte, at a very early stage of its development
becomes free from the gametophyte and is entirely self-supporting.
Reproduction is effected as in the Bryophyta by sexual reproductive
organs and by asexual methods. Not only have we in the Pteridophytes a
much more complete external division of the plant-body into definite
members, which subserve distinct functions, and behave as well-defined
physiological organs adapted for taking a certain share in the
life-functions of the individual, but the internal differentiation has
reached a much higher stage. True vascular tissue, consisting of xylem
and phloem, occurs for the first time in this class. The whole plant is
traversed by one or more vascular strands composed of xylem and phloem
elements, which are respectively concerned with the distribution of
inorganic and organic food substances.
The Pteridophyta include the most important fossil plants. It is from a
study of the internal structure of various extinct representatives of
this class, that palaeobotanists have been able to contribute facts
of the greatest interest and importance towards the advancement of
botanical science.
The botanist’s chief aim in the anatomical investigation of Palaeozoic
genera is to discover data which point the way to a solution of the
problems of plant-evolution. In the abundant material afforded by the
petrified remnants of ancient floras we have the means of tracing the
past history of existing groups or individual forms, and it is from the
Palaeozoic Pteridophytes that our most valuable results have been so
far obtained.
In this and the following chapters of Volume I. two divisions of
the Pteridophyta are dealt with in such detail as the nature of the
book allows. In the earlier chapters of Volume II. the remaining
representatives of this class will be described. As in the preceding
chapters such recent plants will be described as are most essential for
the correct interpretation of the fossil forms.
It is impossible to do more than confine our attention to a few
only of the genera of living plants which directly concern us;
some acquaintance with the general facts of plant morphology must
be assumed. Among the most useful text-books or books of reference
on the Pteridophyta the student may consult those mentioned in the
footnote[483].
I. EQUISETALES.
Leaves usually small in proportion to the size of the whole plant,
arranged in whorls at the nodes. Sporangia borne on specially modified
sporophylls or sporangiophores, which are aggregated to form a definite
strobilus or spore-bearing cone.
EQUISETACEAE. (Recent Species.)
The leaves are in whorls, coherent in the form of a sheath, and
traversed by longitudinal veins which do not fork or anastomose.
The stem is divided into comparatively long internodes separated by
the leaf-bearing nodes, and the branches arise in the leaf-axils at
the nodes. The fertile leaves or sporophylls differ from the sterile
leaves, and usually occur in definite aggregations or strobili
containing spores of one kind (_isosporous_). In the single living
genus _Equisetum_, the outer coat of the mature spore forms two
hygroscopically sensitive filamentous structures or _elaters_. On the
germination of the spore the gametophyte is developed in the form of
a small lobed prothallium 1–2 cm. in length. In most cases there are
distinct male and female prothallia.
The genus _Equisetum_ L., the common Horse-tail, is the sole living
representative of this Family. It occurs as a common native plant
in Britain, and has a wide geographical distribution. Species of
_Equisetum_ are abundant in the temperate zones of both hemispheres,
and occur in arctic as well as tropical latitudes. Wallace[484]
speaks of Horse-tails, “very like our own species,” growing at a
height of 5000 feet on the Pangerango mountain in Java. In favourable
situations the large British Horse-tail, _Equisetum maximum_ Lam. (=
_E. Telmateia_ Erhb.), occasionally reaches a height of about six feet,
and growing in thick clusters forms miniature forests of trees with
slender erect stems and regular circles of long and thin branches. A
tropical species, _Equisetum giganteum_ Linn.[2] living in the marshes
of Mexico and Cuba[485], and extending southward to Buenos Ayres and
Chili, reaches a height of twenty to forty feet, but the stem always
remains slender, and does not exceed an inch in diameter. Groves of
such tall slender plants on the eastern slopes of the Andes[486]
suggest to the palaeobotanist an enfeebled forest-growth recalling
the arborescent Calamites of a Palaeozoic vegetation. The twenty-five
existing species of _Equisetum_ are remnants of various generic types
of former epochs, and possess a special interest from the point of view
of the geological history of plants. A brief description of the main
characters of the recent genus will enable the student to appreciate
the points of difference and agreement between the extinct and present
representatives of the Equisetales.
[Illustration: FIG. 52. _Equisetum maximum_ Lam. A. Fertile shoot with
strobilus and sterile leaf-sheaths [after Luerssen (89); slightly
less than nat. size]. B. Sporophyll bearing open sporangia (after
Luerssen; slightly enlarged). C. Part of a transverse section
(diagrammatic); _v_, vallecular canals, _e_, endodermis, _c_,
carinal canals (after Luerssen; × 20). D. _Equisetum arvense_ L.
Part of a transverse section of an internode of a sterile shoot.
_v_, cortex, _e_, endodermis, _x_, xylem tracheids, _a_ remains
of annular tracheids of the protoxylem, _c_, carinal canal (after
Strasburger; × 90).]
_Equisetum._
The plant consists of a perennial underground creeping rhizome,
branching into secondary rhizomes, divided into well-marked nodes and
internodes. From the nodes are given off two sets of buds, which may
develope into ascending aerial shoots or descending roots. At each node
is a leaf-sheath more or less deeply divided along the upper margin
into teeth representing the tips of coherent leaves (fig. 52, A).
In some species one or more internodes of underground branches become
considerably swollen and assume the form of ovate or elliptical
starch-storing tubers, which are capable of giving rise to new plants
by vegetative reproduction. Tubers, either singly or in chains, occur
in _E. arvense_ Linn., _E. sylvaticum_ Linn., _E. maximum_ Lam., among
British species.
[Illustration: FIG. 53. Rhizome (R) of _Equisetum palustre_ L. with a
thin shoot giving off roots and tuberous branches from a node
[after Duval-Jouve (64)].]
In the example shown in fig. 53 (_Equisetum palustre_ L.[487]) the
stout rhizome R gives off from its node, marked by a small and
irregular leaf-sheath, two thin roots and a single shoot. The latter
has a leaf-sheath at its base, and from the second node, with a
larger leaf-sheath, there have been developed branches with tuberous
internodes; the constrictions between the tubers and the tips of the
terminal tubers bear small leaf-sheaths. Branched roots are also given
off from the upper node of the erect shoot.
Near the surface of the ground the buds on the rhizome nodes develope
into green erect shoots. The shoot axis is marked out into long
internodes separated by nodes bearing the leaf-sheaths. The surface
of each internode is traversed by regular and more or less prominent
longitudinal ridges and grooves; each ridge marking the position of
an internal longitudinal vascular strand. In the axil of each leaf,
that is in the axil of each portion of a leaf-sheath corresponding to
a marginal uni-nerved tooth, there is produced a lateral bud which may
either remain dormant or break through the leaf-sheath and emerge as
a lateral branch. At the base of each branch an adventitious root may
be formed from a cell immediately below the first leaf-sheath, but
in aerial shoots the roots usually remain undeveloped. The lateral
branches repeat on a smaller scale the general features of the main
axis. In some species, the shoots are unbranched, and in others the
slender branches arise in crowded whorls from each node. Leaves, roots
and branches are given off in whorls, and the whorls from each node
alternate with those from the node next above and next below.
[Sidenote: ANATOMY OF EQUISETUM.]
In some species of _Equisetum_ the aerial stem terminates in a
conical group of sporophylls, while in others the strobilus is formed
at the apex of a pale-coloured fertile shoot, which never attains
any considerable length and dies down early in the season of growth
(fig. 52, A). Below the terminal cone or strobilus there occur one
or two modified leaf-sheaths. Such a ring of incompletely developed
leaves intervening between the cone of sporangiophores and the normal
leaves, is known as the _annulus_. The annulus is seen in fig. 52, A,
immediately below the lowest whorl of sporophylls; it has the form
of a low sheath with a ragged margin. In the region of the cone the
internodes remain shorter, and the whorls of appendages, known as
sporophylls or sporangiophores, have the form of stalked structures
terminating distally in a hexagonal peltate disc, which bears on its
inner face a ring of five to ten oval sporangia (fig. 52, B). Each
sporangium contains numerous spores which eventually escape by the
longitudinal dehiscence of the sporangial wall. The opening of the
sporangia is probably assisted by the movements of the characteristic
elaters formed from the outer wall of each spore.
The spores, which are capable of living only a short time, grow into
aerial green prothallia, 1–2 cm. in length; these have the form
of irregularly and more or less deeply lobed structures. On the
larger and more deeply lobed prothallia the archegonia or female
reproductive organs are borne, and the smaller or male prothallia bear
the antheridia. On the fertilisation of an egg-cell, the _Equisetum_
plant is gradually developed. For a short time parasitic on the female
prothallium or gametophyte, the young plant soon takes root in the
ground and becomes completely independent.
As seen in transverse section through a young stem near the apex, the
axis consists of a mass of parenchyma, in which may be distinguished a
central larger-celled tissue, surrounded by a ring of smaller-celled
groups marking the position of a circle of embryonic vascular strands.
In each young vascular strand, a few of the cells next the pith may be
seen to have thicker walls and to be provided with a ring-like internal
thickening; these have passed over into the condition of annular
tracheids and represent the _protoxylem_ elements. At a later stage, a
transverse section through the stem shows a central hollow pith, formed
by the tearing apart and subsequent disappearance of the medullary
parenchymatous cells, which were unable to keep pace with the growth in
thickness of the stem. The pith cavity is bridged across at each node
by a multi-layered plate of parenchyma, which forms the so-called nodal
_diaphragm_. The inner edge of each vascular strand is now found to be
occupied by a small irregularly circular canal (fig. 52, C, c, and D,
c) in which may be seen some of the rings of protoxylem tracheids (D,
a) which have been torn apart and almost completely destroyed. These
canals, known as _carinal canals_, have arisen by the tearing and
disruption of the thin-walled cells in the immediate neighbourhood of
the protoxylem. Each carinal canal is bounded by a layer of elongated
parenchymatous cells which form part of the xylem of the vascular
bundle, and is succeeded internally by the general ground-tissue
of the stem. The xylem parenchyma next a carinal canal is succeeded
externally by phloem tissue, consisting of short protoplasmic cells and
longer elements, without nuclei and poor in contents; the latter may be
regarded as sieve-tubes. On either side of the phloem, the xylem occurs
in two separate bands or groups of annular and reticulately thickened
tracheids. In some species, _e.g._ _Equisetum xylochaetum_ Metten.[488]
and _E. giganteum_[489] L. a native of South America, the xylem has
the form of two bands composed of fairly numerous tracheids, but in
most species the xylem tracheids occur in small groups, as shown in
the figure of _E. maximum_ (fig. 52, D). In the shape of the vascular
bundle, and in the formation of the carinal canal, there is a distinct
resemblance between the vascular bundles of _Equisetum_ and those of
a monocotyledonous stem. These collateral stem-bundles of xylem and
phloem traverse each internode as distinct strands, and at the nodes
each strand forks into two branches (fig. 54, A), which anastomose with
the alternating bundles passing into the stem from the leaf-sheath.
Thus the vascular strands of each internode alternate in position with
those of the next internode.
[Illustration: FIG. 54. A. Plan of the vascular bundles in the stem of
an _Equisetum_; _b_, branches passing out to buds (after
Strasburger); _l_, vascular strands passing to the leaf-segments.
B. Longitudinal section through a node of _E. arvense_ L. (after
Duval-Jouve; × 20). Explanation in the text.]
There are certain points connected with the vascular bundles in the
nodal region of a shoot, which have an important bearing on the
structure of fossil equisetaceous stems. Fig. 54 B represents a
diagrammatic longitudinal section through the node of a rhizome of
_Equisetum arvense_ from which a root _h_ is passing off in a downward
direction, and a branch in an upward direction. The black band _c_
in the parent stem shows the position of the vascular strands; in
the region of the node the vascular tissue attains a considerable
thickness, as seen at _d_ in the figure. The bands passing out to
the left from _d_ go to supply the branch and root respectively. The
increased breadth of the xylem strands at the node is due to the
intercalation of a number of short tracheids. Fig. 55, 4 shows a
transverse section through a mature node of _Equisetum maximum_; _px_
marks the position of the protoxylem and _e_ that of the endodermis.
On comparing this section with that of the internodal vascular bundle
in fig. 52, D, the much greater development of wood in the former is
obvious; the carinal canal of the internodal bundle is absent in the
section through a node. The disposition of the xylem tracheids in fig.
55, 4 shows a certain regularity which, though not very well marked,
suggests the development of wood elements as the result of cambial
activity. Longitudinal sections through the nodal region demonstrate
the existence of “cells similar to those of an ordinary cambium, and a
cell-formation resulting from their division which is similar to that
in an ordinary secondary thickening.”[490] The short tracheids which
make up this nodal mass of xylem differ from those in the internodal
bundle in their smaller size, and in being reticulately thickened.
There is, therefore, evidence that in the nodes of some _Equisetum_
stems additional xylem elements are produced by a method of growth
comparable with the cambial activity which brings about the growth in
thickness of a forest-tree[491]. The significance of these statements
will be realised when the structure of the extinct genus _Calamites_ is
described and compared with that of _Equisetum_.
The small drawing in fig. 55, 3 shows part of the ring of thick
nodal wood; the section cuts through two bundles about their point
of bifurcation, the strand _x_ is passing out in a radial direction
to a lateral branch, the strand to the right of _x_ and the separate
fragment of a strand to the left of _x_ are portions of leaf-trace
bundles on their way to the leaf-sheath. Reverting to fig. 54, B, the
other structures seen in the section are the leaf-sheaths (_l_ and
_m_), the vallecular canal (_f_), the epidermis, cortex and pith (_k_,
_e_ and _a_) of the stem. The epidermis which has been ruptured by the
root and branch is indicated at _i_, _i_; the dotted lines traversing
the upper part of the pith of the lateral branch mark the position of a
nodal diaphragm.
[Illustration: FIG. 55. 1. Transverse section of a root of _Equisetum
variegatum_ Schl., _e_ endodermis, or outer layer of the
phloeoterma (after Pfitzer; × 160). 2. Transverse section of
rhizome of _E. maximum_, slightly enlarged. 3. Transverse section
through a node of _E. maximum_, _x_, branch of vascular strand
(slightly enlarged). 4. Transverse section through a node of _E.
maximum_ showing the mass of xylem, _px_ protoxylem (× 175). (Figs.
3 and 4 after Cormack.)]
Immediately external to each vascular strand, as seen in transverse
section, there is a layer of cells containing starch, and this
is followed by a distinct endodermis, of which the cells show the
characteristic black dot in the cuticularised radial walls (fig. 52,
D). Beyond the endodermis there is the large-celled parenchyma of
the rest of the cortex. Tannin cells occur here and there scattered
among the ground tissue. On the same radius on which each vascular
strand occurs, the cortical parenchyma passes into a mass of
sub-epidermal thick-walled mechanical tissue or stereome. Alternating
with the ridges of stereome, the grooves are occupied by thin-walled
chlorophyll-containing tissue which carries on most of the assimilating
functions, and communicates with the external atmosphere by means of
stomata arranged in vertical rows down each internode. The continuity
of the cortical tissue is interrupted by the occurrence of large
longitudinal _vallecular canals_ alternating in position with the stem
ridges and vascular strands (fig. 52, C, _v_). The epidermis consists
of a single layer of cells, containing stomata, and with the outer
cell-walls impregnated with silica.
In certain species of _Equisetum_, _e.g._ _E. palustre_ L., the whole
circle of vascular strands is enclosed by an endodermis, and has the
structure typical of a monostelic stem. In others _e.g._ _E. litorale_
Kühl. each vascular strand is surrounded by a separate endodermis,
and in some forms _e.g._ _E. sylvaticum_ L. there is an inner as well
as an outer endodermal layer[492]. Without discussing the explanation
given to this variation in the occurrence of the endodermis, it may be
stated that in all species of _Equisetum_ the stem may be regarded as
monostelic[493].
In the rhizome the structure agrees in the main with that of the
green shoots, but the vallecular canals attain a larger size, and the
pith is solid. A slightly enlarged transverse section of a rhizome
of _Equisetum maximum_ is shown in fig. 55, 2, the small circles
surrounding the pith mark the position of the vascular bundles and
carinal canals; the much larger spaces between the central cylinder and
the surface of the stem are the vallecular canals.
The central cylinder or stele of the root is of the diarch, triarch or
tetrarch type; _i.e._ there may be 2, 3 or 4 groups of protoxylem in
the xylem of the root stele. The axial portion is occupied by large
tracheids, and the smaller tracheids of the xylem occur as radially
disposed groups, alternating with groups of phloem. External to the
xylem and phloem strands there occur two layers of cells, usually
spoken of as a double endodermis, but it has been suggested that it
is preferable to describe the double layer as the _phloeoterma_[494],
of which the inner layer has the functions of a pericycle, and the
outer that of an endodermis. A transverse section of a root is seen
in fig. 55, 1, the dark cells on the left are part of a thick band of
sclerenchyma in the cortex of the root, the layer e is the outer layer
of the phloeoterma.
Without describing in detail the development[495] of the sporangia,
it should be noted that the sporangial wall is at first 3 to 4 cells
thick, but it eventually consists of a single layer. The cells have
spiral thickening bands on the ventral surface, and rings on the
cells where the longitudinal splitting takes place. Each sporangium
is supplied by a vascular bundle which is given off from that of the
sporangiophore axis. The strobili are isosporous.
╭ A. EQUISETITES.
│ B. PHYLLOTHECA.
II. FOSSIL EQUISETALES. ┤ C. SCHIZONEURA.
│ D. CALAMITES.
╰ E. ARCHAEOCALAMITES.
In dealing with the fossil Equisetales, we will first consider
the genera _Equisetites_, _Phyllotheca_ and _Schizoneura_, and
afterwards describe the older and better known genera _Calamites_
and _Archaeocalamites_. A thoroughly satisfactory classification of
the members of the Equisetales is practically impossible without
more data than we at present possess. It has been the custom to
include _Equisetites_, _Phyllotheca_ and _Schizoneura_ in the family
Equisetaceae, and to refer _Calamites_ and _Archaeocalamites_ to the
Calamarieae; such a division rests in part on assumption, and cannot
be considered final. When we attempt to define the Equisetales and the
two families Equisetaceae and Calamarieae, we find ourselves seriously
hampered by lack of knowledge of certain important characters, which
should be taken into account in framing diagnoses. There is little
harm in retaining provisionally the two families already referred to,
if we do not allow a purely arbitrary classification to prejudice our
opinions as to the affinities of the several members of the Equisetales.
The Equisetaceae might be defined as a family including plants which
were usually herbaceous but in some cases arborescent, bearing
verticils of leaves in the form of sheaths more or less deeply divided
into segments or teeth. The strobili were isosporous and consisted of
a central axis bearing verticils of distally expanded sporophylls with
sporangia, as in _Equisetum_. The genus _Equisetites_ might be included
in this family, but it must be admitted that we know next to nothing
as to its anatomy, and we cannot be sure that the strobili were always
isosporous.
The genus _Schizoneura_ is too imperfectly known to be defined with any
approach to completeness, or to be assigned to a family defined within
certain prescribed limits. _Phyllotheca_ is another genus about which
we possess but little satisfactory knowledge; we are still without
evidence as to its structure, and the descriptions of the few strobili
that are known are not consistent. Recent work points to a probability
of _Phyllotheca_ being closely allied to _Annularia_, a genus included
in the Calamarieae, and standing for a certain type of Calamitean
foliage-shoots.
In comparing the Calamarieae with the Equisetaceae, the alternation of
sterile and fertile whorls in the strobilus, and the free linear leaves
at the nodes instead of leaf-sheaths are two characters made use of as
distinguishing features of the genus _Calamites_ as the type of the
Calamarieae. On the other hand, the strobili of _Phyllotheca_ appear to
agree with those of _Calamites_ rather than with those of _Equisetum_,
and strobili of _Archaeocalamites_ have been found exhibiting the
typical _Equisetum_ characters. The sheath-like form of the leaves
is not necessarily peculiar to the Equisetaceae, and we have evidence
that leaf-sheaths occurred on the nodes of Calamitean plants. In
_Archaeocalamites_ the leaves possess characteristic features, and can
hardly be said to agree more closely with those of Calamites than with
the leaves of _Phyllotheca_ or _Sphenophyllum_, a genus belonging to
another class of Pteridophytes.
On the whole, then, without discussing further the possibilities of a
subdivision of the Equisetales, we may regard the genera _Calamites_,
_Archaeocalamites_, _Equisetites_, _Equisetum_, _Phyllotheca_ and
_Schizoneura_ as so many members of the Equisetales, without insisting
on a classification which cannot be supported by satisfactory evidence.
Our knowledge of _Calamites_ is fairly complete. Abundant and
well-preserved material from the Coal-Measures of England, and from
Permo-Carboniferous rocks of France, Germany and elsewhere, has enabled
palaeobotanists to investigate the anatomical characters of both the
vegetative and reproductive structures of this genus. We are in a
position to give a detailed diagnosis of Calamitean stems, roots and
strobili, and to determine the place of this type of plant in a system
of classification. _Calamites_ not only illustrates the possibilities
of palaeobotanical research, but it demonstrates the importance of
fossil forms as foundations on which to construct the most rational
classification of existing plants. The close alliance between
_Calamites_ and the recent Equisetaceae has been clearly established,
and certain characteristics of the former genus render necessary an
extension and modification of the definition of the class to which
both _Calamites_ and _Equisetites_ belong. The Calamites broaden our
conception of the Equisetaceous alliance, and by their resemblance
to other extinct Palaeozoic types they furnish us with important
links towards a phylogenetic series, which the other members of the
Equisetales do not supply.
From the Upper Devonian to the Permian epoch _Calamites_ and other
closely related types played a prominent part in the vegetation of
the world. We have no good evidence for the existence of _Calamites_
in Triassic times; in its place there were gigantic Equisetums which
resembled modern Horse-tails in a remarkable degree. In the succeeding
Jurassic period tree-like Equisetums were still in existence, and
species of _Equisetites_ are met with in rocks of this age in nearly
all parts of the world. A few widely distributed species are known from
Wealden rocks, but as we ascend the geologic series from the Jurassic
strata, the Equisetums become less numerous and the individual plants
gradually assume proportions practically identical with those of
existing forms.
A. _Equisetites._
The generic name _Equisetites_ was proposed by Sternberg in
1838[496] as a convenient designation for fossil stems bearing a
close resemblance to recent species of _Equisetum_. Some authors
have preferred to apply the name _Equisetum_ to fossil and recent
species alike, but in spite of the apparent identity in the external
characters of the fossil stems with those of existing Horse-tails, and
a close similarity as regards the cones, there are certain reasons
for retaining Sternberg’s generic name. It is important to avoid such
nomenclature as might appear to express more than the facts admit. If
the custom of adding the termination -_ites_ to the root of a recent
generic term is generally followed, it at once serves to show that the
plants so named are fossil and not recent species. Moreover, in the
case of fossil Equisetums we know nothing of their internal structure,
and our comparisons are limited to external characters. Stems, cones,
tubers, and leaves are often very well preserved as sandstone casts
with distinct surface-markings, but we are still in want of petrified
specimens. There is indeed evidence that some of the Triassic and
Jurassic species of _Equisetites_, like the older Calamites, possessed
the power of secondary growth in thickness, but our deductions are
based solely on external characters.
In the following pages a few of the better known species of
_Equisetites_ are briefly described, the examples being chosen partly
with a view to illustrate the geological history of the genus, and
partly to contribute something towards a fuller knowledge of particular
species. One of the most striking facts to be gleaned from a general
survey of the past history of the Equisetaceae is the persistence since
the latter part of the Palaeozoic period of that type of plant which
is represented by existing Equisetums. There is perhaps no genus in
existence which illustrates more vividly than _Equisetum_ the survival
of an extremely ancient group, which is represented to-day by numerous
and widely spread species. The Equisetaceous characteristics mark
an isolated division of existing Vascular Cryptogams, and without
reference to extinct types it is practically impossible to do more than
vaguely guess at the genealogical connections of the family. When we go
back to Palaeozoic plants there are indications of guiding lines which
point the way to connecting branches between the older Equisetales
and other classes of Pteridophytes. The recently discovered genus
_Cheirostrobus_[497] is especially important from this point of view.
[Sidenote: LEAF-SHEATHS OF EQUISETITES.]
The accurate description of species, and the determination of the value
of such differences as are exhibited in the surface characters of
structureless casts, are practically impossible in many of the fossil
forms. In certain living Horse-tails we find striking differences
between fertile and sterile shoots, and between branches of different
orders. The isolated occurrence of fragments of fossil stems often
leads to an artificial separation of ‘species’ largely founded on
differences in diameter, or on slight variations in the form of the
leaf-sheaths. It is wiser to admit that in many cases we are without
the means of accurate diagnosis, and that the specific names applied
to fossil Equisetums do not always possess much value as criteria of
taxonomic differences.
The specimens of fossil Equisetums are usually readily recognised by
the coherent leaf-segments in the form of nodal sheaths resembling
those of recent species. The tissues of the cortex and central
cylinder are occasionally represented by a thin layer of coal pressed
on to the surface of a sandstone cast, or covering a flattened
stem-impression on a piece of shale. It is sometimes possible under
the microscope to recognise on the carbonised epidermal tissues
the remains of a surface-ornamentation similar to that in recent
species, which is due to the occurrence of siliceous patches on the
superficial cells. Longitudinal rows of stomata may also be detected
under favourable conditions of preservation. The nodal diaphragms
of stems have occasionally been preserved apart, but such circular
and radially-striated bodies may be misleading if found as isolated
objects. Casts of the wide hollow pith of _Equisetites_, with
longitudinal ridges and grooves, and fairly deep nodal constrictions,
have often been mistaken for the medullary casts of _Calamites_.
Several species of _Equisetites_ have been recorded from the Upper
Coal-Measures and overlying Permian rocks, but these present special
difficulties. In one instance described below, (_Equisetites
Hemingwayi_ Kidst.), the species was founded on a cast of what appeared
to be a strobilus made up of sporophylls similar to those in an
_Equisetum_ cone. In other Permo-Carboniferous species the choice of
the generic name _Equisetites_ has been determined by the occurrence
of leaf-sheaths either isolated or attached to the node of a stem. The
question to consider is, how far may the Equisetum-like leaf-sheath
be regarded as a characteristic feature of _Equisetites_ as distinct
from _Calamites_? In the genus _Calamites_ the leaves are generally
described as simple linear leaves arranged in a whorl at the nodes,
but not coherent in the form of a sheath (fig. 85). The fusion of
the segments into a continuous sheath or collar is regarded as a
distinguishing characteristic of _Equisetites_ and _Equisetum_. The
typical leaf-sheath of a recent Horse-tail has already been described.
In some species we have fairly large and persistent free teeth on the
upper margin of the leaf-sheath, but in other Equisetums the rim of
the sheath is practically straight and has a truncated appearance,
the distal ends of the segments being separated from one another by
very slight depressions, as in a portion of the sheath of _Equisetum
ramosissimum_ Desf. of fig. 58, _C_. In other leaf-sheaths of this
species there are delicate and pointed teeth adherent to the margin
of the coherent segments; the teeth are deciduous, and after they have
fallen the sheath presents a truncated appearance. This difference
between the sheaths to which the teeth are still attached and those
from which they have fallen is illustrated by fig. 58, _B_ and _C_;
it is one which should be borne in mind in the description of fossil
species, and has probably been responsible for erroneous specific
diagnoses. In some recent Horse-tails the sheath is occasionally
divided in one or two places by a slit reaching to the base of the
coherent segments[498]; this shows a tendency of the segments towards
the free manner of occurrence which is usually considered a Calamitean
character. In certain fossils referred to the genus _Annularia_, the
nodes bear whorls of long and narrow leaves which are fused basally
into a collar (fig. 58, _D_). There are good grounds for believing that
at least some Annularias were the foliage shoots of true Calamites.
Again, in some species of _Calamitina_, a sub-genus of _Calamites_,
the leaves appear to have been united basally into a narrow sheath.
We see, then, that it is a mistake to attach great importance to the
separate or coherent character of leaf-segments in attempting to draw
a line between the true _Calamites_ and _Equisetites_. Potonié[499]
while pointing out that this distinction does not possess much value
as a generic character, retains the genus _Equisetites_ for certain
Palaeozoic Equisetum-like leaf-sheaths.
[Illustration: FIG. 56. Calamitean leaf-sheath. From a specimen in the
Woodwardian Museum. _a_, base of leaf-sheath; (very slightly
reduced).]
Fig. 56 represents a rather faint impression of a leaf-sheath and
nodal diaphragm. The specimen is from the Coal-Measures of Ardwick,
Manchester. The letter _a_ probably points to the attachment of the
sheath to the node of the stem. The flattened sheath is indistinctly
divided into segments, and at the middle of the free margin there
appears to be a single free tooth. The lower part of the specimen, as
seen in the figure, shows the position of the nodal diaphragm. Between
the diaphragm and the sheath there are several slight ridges converging
towards the nodal line; these agree with the characteristic ridges and
grooves of Calamite casts which are described in detail in Chapter X.
There is another specimen in the British Museum which illustrates,
rather more clearly than that shown in fig. 56, the association of a
fused leaf-sheath with a type of cast usually regarded as belonging
to a Calamitean stem. Some leaf-sheaths of Permian age described by
Zeiller[500] as _Equisetites Vaujolyi_ bear a close resemblance to
the sheath in fig. 58 E. The nature of the true Calamite leaves is
considered more fully on a later page.
[Sidenote: PALAEOZOIC EQUISETITES.]
The examples of supposed _Equisetites_ sheaths referred to below may
serve to illustrate the kind of evidence on which this genus has
been recorded from Upper Palaeozoic rocks. I have retained the name
_Equisetites_ in the description of the species, but it would probably
be better to speak of such specimens as ‘Calamitean leaf-sheaths’
rather than to describe them as definite species of _Equisetites_.
We have not as yet any thoroughly satisfactory evidence that the
_Equisetites_ of Triassic and post-Triassic times existed in the
vegetation of earlier periods.
In Grand’Eury’s _Flore du Gard_[501] a fossil strobilus is figured
under the name _Calamostachys tenuissima_ Grand’Eury, which consists of
a slender axis bearing series of sporophylls and sporangia apparently
resembling those of an _Equisetum_. There are no sterile appendages or
bracts alternating with the sporophylls; and the absence of the former
suggests a comparison with _Equisetites_ rather than _Calamites_.
Grand’Eury refers to the fossil as “parfois à peine perceptible,” and a
recent examination of the specimen leads me to thoroughly endorse this
description. It was impossible to recognise the features represented
in Grand’Eury’s drawing. Setting aside this fossil, there are other
strobili recorded by Renault[502] and referred by him to the genus
_Bornia_ (_Archaeocalamites_), which also exhibit the Equisetum-like
character; the axis bears sporophylls only and no sterile bracts. It
would appear then that in the Palaeozoic period the Equisetaceous
strobilus, as we know it in _Equisetum_, was represented in some of the
members of the Equisetales.
[Illustration: FIG. 57. A. _Equisetites Hemingwayi_ Kidst. From a
specimen in the British Museum. ⅔ nat. size. B. Diaphragm and
sheath of an Equisetaceous plant, from the Coal-Measures. ⅔ nat.
size. From a specimen in the British Museum.]
1. _Equisetites Hemingwayi_ Kidst. Fig. 57, _A_.
Mr Kidston[503] founded this species on a few specimens of cones
found in the Middle Coal-Measures of Barnsley in Yorkshire. The best
example of the cone described by Kidston has a length of 2·5 cm.,
and a breadth of 1·5 cm.; the surface is divided up into several
hexagonal areas 4 mm. high and 5 mm. wide. Each of these plates shows
a fairly prominent projecting point in its centre; this is regarded
as the point of attachment of the sporangiophore axis which expanded
distally into a hexagonal plate bearing sporangia. An examination of
Mr Kidston’s specimens enabled me to recognise the close resemblance
which he insists on between the fossils and such a recent Equisetaceous
strobilus as that of _Equisetum limosum_ Sm. Nothing is known of the
structure of the fossils beyond the character of the superficial
pattern of the impressions, and it is impossible to speak with absolute
confidence as to their nature. The author of the species makes
use of the generic name _Equisetum_; but in view of our ignorance
of structural features it is better to adopt the more usual term
_Equisetites_.
Since Kidston’s description was published I noticed a specimen in the
British Museum collection which throws some further light on this
doubtful fossil. Part of this specimen is shown in fig. 57, _A_. The
stem is 21 cm. in length and about 5 mm. broad; it is divided into
distinct nodes and internodes; the former being a little exaggerated in
the drawing. The surface is marked by fine and irregular striations,
and in one or two places there occur broken pieces of narrow linear
leaves in the neighbourhood of a node. Portions of four cones occurring
in contact with the stem, appear to be sessile on the nodes, but
the preservation is not sufficiently good to enable one to speak
with certainty as to the manner of attachment. Each cone consists of
regular hexagonal depressions, which agree exactly with the surface
characters of Kidston’s type-specimen. The manner of occurrence of the
cones points to a lateral and not a terminal attachment. The stem does
not show any traces of Equisetaceous leaf-sheaths at the nodes, and
such fragments of leaves as occur appear to have the form of separate
linear segments; they are not such as are met with on _Equisetites_.
It agrees with some of the slender foliage-shoots of Calamitean plants
often described under the generic name _Asterophyllites_. As regards
the cones; they differ from the known Calamitean strobili in the
absence of sterile bracts, and appear to consist entirely of distally
expanded sporophylls as in _Equisetum_. The general impression afforded
by the fossil is that we have not sufficient evidence for definitely
associating this stem and cones with a true _Equisetites_. We may,
however, adhere to this generic title until more satisfactory data are
available.
2. _Equisetites spatulatus_ Zeill. Fig. 58, _A_.
This species is chosen as an example of a French _Equisetites_ of
Permian age. It was recently founded by Zeiller[504] on some specimens
of imperfect leaf-sheaths, and defined as follows:—
Sheaths spreading, erect, formed of numerous uninerved coherent
leaves, convex on the dorsal surface, spatulate in form, 5–6 cm.
in length and 2–3 mm. broad at the base, and 5–10 mm. broad at the
apex, rounded at the distal end.
The specimen shown in fig. 58, _A_, represents part of a flattened
sheath, the narrower crenulated end being the base of the sheath.
The limits of the coherent segments and the position of the veins
are clearly marked. Zeiller’s description accurately represents the
character of the sheaths. They agree closely with an Equisetaceous
leaf-sheath, but as I have already pointed out, we cannot feel certain
that sheaths of this kind were not originally attached to a Calamite
stem.
The portion of a leaf-sheath and a diaphragm represented in fig. 57,
_B_, agrees closely with Zeiller’s examples. This specimen is from
the English Coal-Measures, but it is not advisable to attempt any
specific diagnosis on such fragmentary material. It is questionable,
indeed, if these detached fossil leaf-sheaths should be designated by
specific names. Another similar form of sheath, hardly distinguishable
from Zeiller’s species, has recently been described by Potonié from the
Permian (Rothliegende) of Thuringia.
[Illustration: FIG. 58.
_A._ _Equisetites spatulatus_, Zeill. Leaf-sheath. ⅘ nat. size.
(After Zeiller.)
_B._ _E. columnaris_, Brongn. From a specimen in the British Museum.
¾ nat. size.
_C._ _Equisetum ramosissimum_, Desf. × 2.
_D._ _Annularia stellata_ (Schloth.). Leaf-sheath. Slightly enlarged.
(After Potonié.)
_E._ _Equisetites zeaeformis_ (Schloth.). Leaf-sheath. ⅘ nat. size.
(After Potonié.)
_F._ _E. lateralis_, Phill. From a specimen in the Scarborough
Museum. Nat. size.]
3. _Equisetites zeaeformis_ (Schloth.)[505] Fig. 58, _E_.
The sheaths consist of linear segments fused laterally as in
_Equisetum_. In some specimens the component parts of the sheath are
more or less separate from one another, and in this form they are
apparently identical with the leaves of _Calamites_ (_Calamitina_)
_varians_, Sternb. The example shown in fig. 58, _E_ is probably a
young leaf-sheath; the segments are fused, and each is traversed by
a single vein represented by a dark line in the figure. The regular
crenulated lower margin is the base of the sheath, and corresponds to
the upper portion of fig. 58, _A_. This species affords, therefore, an
interesting illustration of the difficulty of separating _Equisetites_
leaves from those of true _Calamites_. Potonié has suggested that the
leaf-sheath of a young Calamite might well be split up into distinct
linear segments as the result of the increase in girth of the stem.
• • • • •
Other Palaeozoic species of _Equisetites_ have been recorded, but with
one exception these need not be dealt with, as they do not add anything
to our knowledge of botanical importance. The specimen described
in the _Flore de Commentry_ as _Equisetites Monyi_, by Renault and
Zeiller[506], differs from most of the other Palaeozoic species of
_Equisetites_, in the fact that we have a stem with short internodes
bearing a leaf-sheath at each node divided into comparatively long
and distinct teeth. This species presents a close agreement with
specimens of _Calamitina_, but Renault and Zeiller consider that it is
generically distinct. They suggest that the English species, originally
described and figured by Lindley and Hutton[507] as _Hippurites
gigantea_, and now usually spoken of as _Calamitina_, should be
named _Equisetites_. It would probably be better to adopt the name
_Calamitina_ for the French species. The type-specimen of this species
is in the Natural History Museum, Paris.
[Sidenote: EQUISETITES PLATYODON.]
[Illustration: FIG. 59. _Equisetites platyodon_ Brongn. (After
Schoenlein, slightly reduced.)]
When we pass from the Permian to the Triassic period, we find large
casts of very modern-looking Equisetaceous stems which must clearly be
referred to the genus _Equisetites_. The portion of a stem represented
in fig. 59 known as _Equisetites platyodon_ Brongn.[508] affords an
example of a Triassic Equisetaceous stem with a clearly preserved
leaf-sheath. The stem measures about 6 cm. in diameter. One of the
oldest known Triassic species is _Equisetites Mougeoti_[509] (Brongn.)
from the Bunter series of the Vosges.
The Keuper species _E. arenaceus_ is, however, more completely known.
The specimens referred to this species are very striking fossils; they
agree in all external characters with recent Horse-tails but greatly
exceed them in dimensions.
4. _Equisetites arenaceus_ Bronn.
This plant has been found in the Triassic rocks of various parts of
Germany and France; it occurs in the Lettenkohl group (Lower Keuper),
as well as in the Middle Keuper of Stuttgart and elsewhere. The species
may be defined as follows:—
Rhizome from 8–14 cm. in diameter, with short internodes,
bearing lateral ovate tubers. Aerial shoots from 4–12 cm. in
diameter, bearing whorls of branches, and leaf-sheaths made up
of 110–120 coherent uni-nerved linear segments terminating in an
apical lanceolate tooth. Strobili oval, consisting of crowded
sporangiophores with pentagonal and hexagonal peltate terminations.
The casts of branches, rhizomes, tubers, buds and cones enable us to
form a fairly exact estimate of the size and general appearance of
this largest fossil Horse-tail. The Strassburg Museum contains many
good examples of this species, and a few specimens may be seen in
the British Museum. In the École des Mines, Paris, there are some
exceptionally clear impressions of cones of this species from a lignite
mine in the Vosges.
It is estimated that the plant reached a height of 8 to 10 meters,
about equal to that of the tallest recent species of _Equisetum_,
but in the diameter of the stems the Triassic plant far exceeded any
existing species.
It is interesting to determine as far as possible, in the absence of
petrified specimens, if this Keuper species increased in girth by
means of a cambium. There are occasionally found sandstone casts of
the pith-cavity which present an appearance very similar to that of
Calamitean medullary casts[510]. The nodes are marked by comparatively
deep constrictions, which probably represent the projecting nodal wood.
The surface of the casts is traversed by regular ridges and grooves
as in an ordinary Calamite, and it is probable that in _Equisetites
arenaceus_, as in _Calamites_, these surface-features are the
impression of the inner face of a cylinder of secondary wood (_cf._
p. 310). Excellent figures of this species of _Equisetites_ are given
by Schimper in his Atlas of fossil plants[511], also by Schimper and
Koechlin-Schlumberger[512], and by Schoenlein and Schenk[513].
5. _Equisetites columnaris_ Brongn. Figs. 11 and 58, _B_.
This species, which is by far the best known British _Equisetites_, was
founded by Brongniart[514] on some specimens from the Lower Oolite beds
of the Yorkshire coast. Casts of stems are familiar to those who have
collected fossils on the coast between Whitby and Scarborough; they
are often found in an erect position in the sandstone, and are usually
described as occurring in the actual place of growth. As previously
pointed out (p. 72), such stems have generally been deposited by water,
and have assumed a vertical position (fig. 11). Young and Bird[515]
figured a specimen of this species in 1822, and in view of its striking
resemblance to the sugar-cane, they regarded the fossil as being of the
same family as _Saccharum officinarum_, if not specifically identical.
A specimen was described by König[516] in 1829, from the Lower
Oolite rocks of Brora in the north of Scotland under the name of
_Oncylogonatum carbonarium_, but Brongniart[517] pointed out its
identity with the English species _Equisetites columnaris_.
Our acquaintance with this species is practically limited to the
casts of stems. A typical stem of _E. columnaris_ measures 3 to 6
cm. in diameter and has fairly long internodes. The largest stem in
the British Museum collection has internodes about 14 cm. long and a
diameter of about 5 cm. In some cases the stem casts show irregular
lateral projections in the neighbourhood of a node, but there is no
evidence that the aerial shoots of this species gave off verticils of
branches. In habit _E. columnaris_ probably closely resembled such
recent species as _Equisetum hiemale_ L., _E. trachyodon_ A. Br. and
others.
The stems often show a distinct swelling at the nodes; this may be due,
at least in part, to the existence of transverse nodal diaphragms which
enabled the dead shoots to resist contraction in the region of the
nodes. The leaf-sheaths consist of numerous long and narrow segments
often truncated distally, as in fig. 58, _B_, and as in the sheath of
such a recent Horse-tail as _E. ramosissimum_ shown in fig. 58, _C_. In
some specimens one occasionally finds indications of delicate acuminate
teeth extending above the limits of a truncated sheath. Brongniart
speaks of the existence of caducous acuminate teeth in his diagnosis of
the species, and the example represented in fig. 58, _B_, demonstrates
the existence of such deciduous appendages. There is a very close
resemblance between the fossil sheath of fig. 58, _B_, with and without
the teeth, and the leaf-sheath of the recent _Equisetum_ in fig. 58,
_C_. In some specimens of _E. columnaris_ in which the cast is covered
with a carbonaceous film, each segment in a leaf-sheath is seen to be
slightly depressed in the median portion, which is often distinctly
marked by numerous small dots, the edges of the segment being flat and
smooth. The median region is that in which the stomata are found and on
which deposits of silica occur.
6. _Equisetites Beani_ (Bunb.). Figs. 60–62.
Bunbury[518] proposed the name _Calamites Beani_ for some fossil stems
from the Lower Oolite beds of the Yorkshire coast, which Bean had
previously referred to in unpublished notes as _C. giganteus_. The
latter name was not adopted by Bunbury on account of the possible
confusion between this species and the Palaeozoic species _Calamites
gigas_ Brongn. The generic name _Calamites_ must be replaced by
_Equisetites_ now that we are familiar with more perfect specimens
which demonstrate the Equisetean characters of the plant.
[Illustration: FIG. 60. _Equisetites Beani_ (Bunb.). ⅔ nat. size.
[After Starkie Gardner (86) Pl. IX. fig. 2.]]
Schimper[519] speaks of this species as possibly the pith-cast of
_Equisetites columnaris_, but his opinion cannot be maintained; the
species first described by Bunbury has considerably larger stems than
those of _E. columnaris_. It is not impossible, however, that _E.
columnaris_ and _E. Beani_ may be portions of the same species. The
chief difference between these forms is that of size; but we have not
sufficient data to justify the inclusion of both forms under one name.
Zigno[520], in his work on the Oolitic Flora, figures an imperfect stem
cast of _E. Beani_ under the name of _Calamites Beani_, but the species
has received little attention at the hands of recent writers. In
1886 Starkie Gardner[521] figured a specimen which was identified by
Williamson as an example of Bunbury’s species; but the latter pointed
out the greater resemblance, as regards the external appearance of the
Jurassic stem, to some of the recent arborescent Gramineae[522] than to
the Equisetaceae. Williamson, with his usual caution, adds that such
appearances have very little taxonomic value. Fig. 60 is reproduced
from the block used by Gardner in his memoir on Mesozoic Angiosperms;
he quotes the specimen as possibly a Monocotyledonous stem. The
fossil is an imperfect cast of a stem showing two clearly marked
nodal regions, but no trace of leaf-sheaths. A recent examination
of specimens in the museums of Whitby, Scarborough, York and London
has convinced me that the plant named by Bunbury _Calamites Beani_
is a large _Equisetites_. As a rule the specimens do not show any
indications of the leaf-sheaths, but in a few cases the sheaths have
left fairly distinct impressions.
[Illustration: FIG. 61. _Equisetites Beani_ (Bunb.). From a specimen in
the British Museum, ⅔ nat. size. (No. V. 2725.)]
In the portion of stem shown in fig. 61 the impressions of the leaf
segments are clearly marked. This specimen affords much better evidence
of the Equisetaceous character of the plant than those which are simply
internal casts. The narrow projecting lines extending upwards from the
nodes in the figured specimen probably represent the divisions between
the several segments of each leaf-sheath.
In the museums of Whitby and Scarborough there are some long specimens,
in one case 44 cm. in length, and 33 cm. in circumference, which
are probably casts of the broad pith-cavity. These casts are often
transversely broken across at the nodes, so that they consist of three
or four separate pieces which fit together by clean-cut faces. This
manner of occurrence is most probably due to the existence of large and
resistant nodal diaphragms which separated the sand-casts of adjacent
internodes. In the York museum there are some large diaphragms, 10
cm. in diameter, preserved separately in a piece of rock containing
a cast of _Equisetites Beani_. The nodal diaphragms of some of the
Carboniferous Calamites were the seat of cork development[523], and
it may be that the frequent preservation of Equisetaceous diaphragms
in Triassic and Jurassic rocks is due to the protection afforded by a
corky investment.
The stem shown in fig. 62 appears to be a portion of a shoot of _E.
Beani_ not far from its apical region. From the lower nodes there
extend clearly marked and regular lines or slight grooves tapering
gradually towards the next higher node; these are no doubt the
impressions of segments of leaf-sheaths. The sheaths themselves have
been detached and only their impressions remain. The flattened bands at
the node of the stem in fig. 60, and shown also in fig. 61, mark the
place of attachment of the leaf-sheaths. On some of these nodal bands
one is able to recognise small scars which are most likely the casts of
outgoing leaf-trace bundles.
Some of the internal casts of this species are marked by numerous
closely arranged longitudinal lines, which are probably the impressions
of the inner face of a central woody cylinder. In the smaller specimen
shown in fig. 62 we have the apical portion of a shoot in which the
uppermost internodes are in an unexpanded condition.
[Illustration: FIG. 62. _Equisetites Beani_ (Bunb.). From a specimen in
the Scarborough Museum. Very slightly reduced.]
It is impossible to give a satisfactory diagnosis of this species
without better material. The plant is characterised chiefly by the
great breadth of the stem, and by the possession of leaf-sheaths
consisting of numerous long and narrow segments. _Equisetites Beani_
must have almost equalled in size the Triassic species, _E. arenaceus_,
described above.
7. _Equisetites lateralis_ Phill. Figs. 58, _F_, 63, and 64.
This species is described at some length as affording a useful
illustration of the misleading character of certain features which
are entirely due to methods of preservation. The specific name was
proposed by Phillips in his first edition of the _Geology of the
Yorkshire Coast_ for some very imperfect stems from the Lower Oolite
rocks near Whitby[524]. The choice of the term _lateralis_ illustrates
a misconception; it was given to the plant in the belief that certain
characteristic wheel-like marks on the stems were the scars of
branches. Lindley and Hutton[525] figured a specimen of this species
in their _Fossil Flora_, and quoted a remark by “Mr Williamson junior”
(afterwards Prof. Williamson) that the so-called scars often occur
as isolated discs in the neighbourhood of the stems. Bunbury[526]
described an example of the same species with narrow spreading leaves
like those of a Palaeozoic _Asterophyllites_, and proposed this generic
name as more appropriate than _Equisetites_. In all probability
the example shown in fig. 63 is that which Bunbury described. It
is certainly the same as one figured by Zigno[527] as _Calamites
lateralis_ in his _Flora fossilis formationis Oolithicae_.
[Illustration: FIG. 63. _Equisetites lateralis_ Phill. From a specimen
in the British Museum. Slightly reduced.]
This specimen illustrates a further misconception in the diagnosis of
the species. The long linear appendages spreading from the nodes are,
I believe, slender branches and not leaves; they have not the form
of delicate filmy markings on the rock face, but are comparatively
thick and almost woody in appearance. The true leaves are distinctly
indicated at the nodes, and exhibit the ordinary features of toothed
sheaths.
Heer[528] proposed to transfer Phillips’ species to the genus
_Phyllotheca_, and Schimper[529] preferred the generic term
_Schizoneura_. The suggestion for the use of these two names would
probably not have been made had the presence of the _Equisetum_ sheaths
been recognised.
The circular depressions a short distance above each node are the
‘branch scars’ of various writers. Schimper suggested that these
radially marked circles might be displaced nodal diaphragms.
Andrae[530] figured the same objects in 1853 but regarded them as
branch scars, although in the specimen he describes, there are several
of them lying apart from the stems, and to one of them is attached
a portion of a leaf-sheath. Solms-Laubach[531] points out that the
internodal position of these supposed scars is an obvious difficulty;
we should not expect to find branches arising from an internode. After
referring to some specimens in the Oxford museum, he adds—“In presence
of these facts the usual explanation of these structures appears to
me, as to Heer, very doubtful.... We are driven to the very arbitrary
assumption that they represent the lowest nodes of the lateral
branches which were inserted above the line of the nodes of the stem.”
Circular discs similar to those of _E. lateralis_ have been found in
the Jurassic rocks of Siberia[532] and elsewhere. There are one or
two examples of such discs from Siberia in the British Museum. If the
nodal diaphragms were fairly hard and stout, it is easy to conceive
that they might have been pressed out of their original position when
the stems were flattened in the process of fossilisation. It is not
quite clear what the radial spoke-like lines of the discs are due to;
possibly they mark the position of bands of more resistant tissue or
of outgoing strands of vascular bundles. A detached diaphragm is seen
in fig. 64 C; in the centre it consists of a flat plate of tissue, and
the peripheral region is traversed by the radiating lines. In the stem
of fig. 64, A the deeply divided leaf-sheaths are clearly seen, and an
imperfect impression of a diaphragm is preserved on the face of the
middle internode. In fig. 64 B a flattened leaf-sheath is shown with
the free acuminate teeth fused basally into a continuous collar[533].
The short piece of stem of _Equisetites lateralis_ shown in fig. 58,
_F_, shows how the free teeth may be outspread in a manner which bears
some resemblance to the leaves of _Phyllotheca_, but a comparison with
the specimens already described, and a careful examination of this
specimen itself, demonstrate the generic identity of the species with
_Equisetites_. The carbonaceous film on the surface of such stems as
those of fig. 58, F, and 64, A, shows a characteristic shagreen texture
which may possibly be due to the presence of silica in the epidermis as
in recent Horse-tails.
There is another species of _Equisetites_, _E. Münsteri_, Schk., from
a lower geological horizon which has been compared with _E. lateralis_,
and lends support to the view that the so-called branch-scars are
nodal diaphragms[534]. This species also affords additional evidence
in favour of retaining the generic name _Equisetites_ for Phillips’
species. _Equisetites Münsteri_ is a typical Rhaetic plant; it has been
found at Beyreuth and Kuhnbach, as well as in Switzerland, Hungary
and elsewhere. A specimen of _Equisetites_ originally described by
Buckman as _E. Brodii_[535], from the Lower Lias of Worcestershire,
may possibly be identical with _E. Münsteri_. The leaf-sheaths of this
Rhaetic species consist of broad segments prolonged into acuminate
teeth; some of the examples figured by Schenk[536] show clearly marked
impressions of displaced nodal diaphragms exactly as in _E. lateralis_.
Another form, _Equisetum rotiferum_ described by Tenison-Woods[537]
from Australia, is closely allied to, or possibly identical with _E.
lateralis_.
[Illustration: FIG. 64. _Equisetites lateralis_ Phill. A. Part of a
stem showing leaf-sheaths and an imperfect diaphragm. B. A single
flattened leaf-sheath. C. A detached nodal diaphragm. From a
specimen in the York Museum. Slightly reduced.]
8. _Equisetites Burchardti_ Dunker[538]. Fig. 65.
This species of _Equisetites_ is fairly common in the Wealden beds of
the Sussex coast near Hastings, and also in Westphalia.
[Illustration: FIG. 65. _Equisetites Burchardti_ Dunk. Showing a node
with two tubers and a root. From a specimen in the British Museum.
Nat. size.]
It is characterised by having long and slender internodes, bearing at
the nodes leaf-sheaths with five or six pointed segments, and by the
frequent formation of branch-tubers. These tuberous branches closely
resemble those which are formed on the underground shoots of _Equisetum
arvense_ L., _E. sylvaticum_ L. and others; they occur either singly
or in chains[539]. In the specimen shown in the figure the left-hand
tuber is remarkably well preserved, its surface is somewhat sunk and
shrivelled, and the apex is surrounded by a nodal leaf-sheath. A thin
branched root is given off just below the point of insertion of the
oval tuber.
No other species of _Equisetites_ affords such numerous examples
of tubers as this Wealden plant. By some of the earlier writers the
detached tubers of _E. Burchardti_ were described as fossil seeds under
the name _Carpolithus_.
[Illustration: FIG. 66. _Equisetites Yokoyamae_ Sew. From specimens in
the British Museum. Nat. size.]
The specimens shown in fig. 66 have been referred to another species,
_E. Yokoyamae_ Sew.[540]; they were obtained from the Wealden beds
of Sussex, but according to Mr Rufford, who discovered them, the
smaller tubers of this species are not found in association with
those of _E. Burchardti_. The stems are very narrow and the tubers
have a characteristic elliptical form; the species is of little value
botanically, but it affords another instance of the common occurrence
of these tuberous branches in the Wealden Equisetums.
Similar fossil tubers, on a much larger scale, have been found in
association with the Triassic _Equisetites arenaceus_; with _E.
Parlatori_ Heer[541], a Tertiary species from Switzerland, and with
other Mesozoic and Tertiary stems. _E. Burejensis_[542], described by
Heer from the Jurassic rocks of Siberia, bears a close resemblance to
the Wealden species.
• • • • •
The description of the above species by no means exhausts the material
which is available towards a history of fossil Equisetums. The
examples which have been selected may serve to illustrate the kind of
specimens that are usually met with, as well as some of the possible
sources of error which have to be borne in mind in the description of
species.
Such Tertiary species as have been recorded need not be considered;
they furnish us with no facts of particular interest from a
morphological point of view. The wide distribution of _Equisetites_,
especially during the Jurassic period, is one of the most interesting
lessons to be learnt from a review of the fossil forms. No doubt a
detailed comparison of the several species from different parts of the
world would lead us to reduce the number of specific names; and at the
same time it would emphasize the apparent identity of fossils which
have been described from widely separated latitudes under different
names.
Specimens of _Equisetites_ are occasionally found in plant-bearing
beds apart from the other members of a Flora; this isolated manner of
occurrence suggests that the plant grew in a different station from
that occupied by Cycads and other elements of the vegetation[543].
A selection of Triassic and Jurassic species arranged in a tabular form
demonstrates the world-wide distribution of this persistent type of
plant[544].
B. _Phyllotheca._
The generic name _Phyllotheca_ was proposed by Brongniart[545] in
1828 for some small fossil stems from the Hawkesbury river, near
Port Jackson, Australia. The stems of this genus are divided into
nodes and internodes and possess leaf-sheaths as in _Equisetum_, but
_Phyllotheca_ differs from other Equisetaceous plants in the form of
the leaves and in the character of its sporophylls. We may define the
genus as follows:—
Plants resembling in habit the recent Equisetums. Stems simple or
branched, divided into distinct nodes and internodes, the latter
marked by longitudinal ridges and grooves; from the nodes are given
off leaf-sheaths consisting of linear-lanceolate uninerved segments
coherent basally, but having the form of free narrow teeth for the
greater part of their length. The long free teeth are usually spread
out in the form of a cup and not adpressed to the stem, the tips of the
teeth are often incurved.
The sporangia are borne on peltate sporangiophores attached to the stem
between whorls of sterile leaves.
Our knowledge of _Phyllotheca_ is unfortunately far from complete. The
chief characteristic of the vegetative shoots consists in the cup-like
leaf-sheaths; these are divided up into several linear segments, which
differ from the teeth of an _Equisetum_ leaf-sheath in their greater
length and in their more open and spreading habit of growth. The large
loose sheaths of the fertile shoots of some recent Horse-tails bear a
certain resemblance to the sheaths of _Phyllotheca_. The diagnosis of
the fertile shoots is founded principally on some Permian specimens of
the genus described by Schmalhausen from Russia[546] and redescribed
more recently by Solms-Laubach[547]. Prof. Zeiller[548] has, however,
lately received some examples of _Phyllotheca_ from the Coal-Measures
of Asia Minor which bear strobili like those of the genus _Annularia_,
a type which is dealt with in the succeeding chapter. A description of
a few species will serve to illustrate the features usually associated
with this generic type, as well as to emphasize the unsatisfactory
state of our knowledge as to the real significance of such supposed
generic characteristics.
There are a few fossil stems from Permian rocks of Siberia,
from Jurassic strata in Italy, and from Lower Mesozoic and
Permo-Carboniferous beds in South America, South Africa, India and
Australia which do not conform in all points to the usually accepted
definition of _Equisetites_, and so justify their inclusion in an
allied genus. On the other hand there are numerous instances of stems
or branches which have been referred to _Phyllotheca_ on insufficient
grounds. Our knowledge of this Equisetaceous plant has recently been
extended by Zeiller[549], who has recorded its occurrence in the
Coal-Measures of Asia Minor associated with typical Upper Carboniferous
plants. The same author[550] has also brought forward good evidence for
the Permian age of the beds in Siberia and Altai, where _Phyllotheca_
has long been known. It is true that Zigno’s species of the genus
occurs in Italian Jurassic rocks, but on the whole it would seem that
this genus is rather a Permian than a Jurassic type. The species
which Zeiller describes under the name _Phyllotheca Rallii_ from
the Coal-Measures of Herakleion (Asia Minor) shows some points of
contact with _Annularia_. It is much to be desired, however, that
we might learn more as to the reproductive organs of this member of
the Equisetales; until we possess a closer acquaintance with the
fructification we cannot hope to arrive at any satisfactory conclusion
as to the exact position of the genus among the Calamarian and
Equisetaceous forms. M. Zeiller[551] informs me that his specimens of
_P. Rallii_, which are to be fully described in a forthcoming work,
include fossil strobili resembling those of _Annularia radiata_.
The verticils of linear leaves fused basally into a sheath agree
in appearance with the star-like leaves of _Annularia_, but in
_Phyllotheca Rallii_ the segments appear to spread in all directions
and are not extended in one plane as in the typical _Annularia_[552].
1. _Phyllotheca deliquescens_ (Göpp.).
In an account of some fossil plants collected by Tchikatcheff in
Altai, Göppert[553] describes and figures two imperfect stems of an
Equisetum-like plant. Owing to the apparent absence of nodal lines on
the surface of the stem the generic name _Anarthrocanna_ is proposed
for the fossils; and the manner in which the main axis appears to break
up into slender branches suggested the specific name _deliquescens_.
Schmalhausen[554] afterwards recognised the generic identity of
Göppert’s fragments with the Indian and Australian stems referred to
the genus _Phyllotheca_ by McCoy[555] and Bunbury[556].
We may define the species as follows:—
Stem reaching a diameter of 2–3 cm. with internodes as much as 4
cm. long, the surface of which is traversed by longitudinal ridges
and grooves which are continuous and not alternate at the nodes.
Branches arise in verticils from the nodes. The leaves have the form
of funnel-shaped sheaths split up into narrow and spreading linear
segments, each of which is traversed by a median vein. The fertile
shoot terminates in a loose strobilus bearing alternating whorls of
sterile bracts and sporangiophores.
The specimens on which this diagnosis is founded are for the most part
fragments of sterile branches. Some of these present the appearance of
Calamitean stems in which the ridges and grooves continue in straight
lines from one internode to the next. Similar stem-casts have been
referred by some writers to the allied genus _Schizoneura_, and it
would appear to be a hopeless task to decide with certainty under
which generic designation such specimens should be described. The
portion of stem shown in fig. 67 affords an example of an Equisetaceous
plant, probably in the form of a cast of a hollow pith, which might
be referred to either _Phyllotheca_ or _Schizoneura_. The specimen
was found in certain South African rocks which are probably of
Permo-Carboniferous age[557]. It agrees closely with some stems from
India described by Feistmantel[558] as _Schizoneura gondwanensis_, and
it also resembles equally closely the Australian specimens referred
by Feistmantel[559] to _Phyllotheca australis_ and some stems of
_Phyllotheca indica_ figured by Bunbury[560].
The longitudinal ridges and grooves shown in fig. 67 probably represent
the broad medullary rays and the projecting wedges of secondary wood
surrounding a large hollow pith, as in _Calamites_. In the Calamitean
casts the ridges and grooves of each internode usually alternate in
position with those of the next, as in _Equisetum_ (fig. 54, A), but in
_Phyllotheca_, _Schizoneura_ and _Archaeocalamites_ there is no such
regular alternation at the nodes of the internodal vascular strands.
[Illustration: FIG. 67. _Phyllotheca?_ ¾ nat. size. From a South
African specimen of Permo-Carboniferous age in the British Museum.]
In _Phyllotheca_ and _Schizoneura_ there are no casts of ‘infranodal
canals’ below each nodal line, but these are by no means always found
in true Calamites. It is therefore practically impossible to determine
the generic position of such fossils as that shown in fig. 67 without
further evidence than is afforded by leafless casts.
A few examples of _Phyllotheca deliquescens_ have been described by
Schmalhausen in which a branch bears clusters of sporangiophores,
alternating with verticils of sterile bracts. The sporangiophores
appear to have the form of stalked peltate appendages bearing
sporangia, very similar to the sporangiophores of _Equisetum_.
Solms-Laubach[561] has examined the best of Schmalhausen’s specimens,
and a carefully drawn figure of one of the fertile branches is given in
his _Fossil Botany_.
The significance of this manner of occurrence of sporangiophores
and whorls of sterile bracts on the fertile branch will be better
understood after a description of the strobilus of _Calamites_. In
_Phyllotheca_ the sporangiophores appear to have been given off in
whorls, which were separated from one another by whorls of sterile
bracts, whereas in _Equisetum_ there are no sterile appendages
associated with the sporangiophores of the strobilus, with the
exception of the annulus at the base of the cone. Heer[562] first drew
attention to the fact that in _Phyllotheca_ we have a form of strobilus
or fertile shoot to a certain extent intermediate in character between
_Equisetum_ and _Calamites_.
In abnormal fertile shoots of _Equisetum_, sporophylls occasionally
occur above and below a sterile leaf-sheath. Potonié[563] has figured
such an example in which an apical strobilus is succeeded at a lower
level by a sterile leaf-sheath, and this again by a second cluster
of sporophylls. As Potonié points out, this alternation of fertile
and sterile members affords an interesting resemblance between
_Phyllotheca_ and _Equisetum_. It suggests a partial reversion towards
the Calamitean type of strobilus.
2. _Phyllotheca Brongniarti_ Zigno. Fig. 68, A.
This species of _Phyllotheca_ from the Lower Oolite rocks of Italy
is known only in the form of sterile branches. The leaves are fused
basally into an open cup-like sheath which is dissected into several
spreading and incurved linear segments. The internodes are striated
longitudinally; they are about 2 mm. in diameter and 10 mm. in length.
The specimen represented in fig. 68, A, was originally described by
the Italian palaeobotanist Zigno[564]; it serves to illustrate the
points of difference between this genus and the ordinary _Equisetum_.
The open and spreading sheaths clasping the nodes and the erect
solitary branches give the plant a distinctive appearance.
[Illustration: FIG. 68.
A. _Phyllotheca Brongniarti_, Zigno. Nat. size. (After Zigno.)
B. _Calamocladus frondosus_, Grand’Eury. (After Grand’Eury.)
Slightly enlarged.
C. _Phyllotheca indica_, Bunb. Part of a leaf-sheath. From a specimen
in the Museum of the Geological Society. Slightly enlarged.]
3. _Phyllotheca indica_ Bunb. and _P. australis_ Brongn. Fig. 68, C.
Sir Charles Bunbury[565] described several imperfect specimens from the
Nagpur district of India under this name, but he expressed the opinion
that it was not clear to him if the plant was specifically distinct
from the _Phyllotheca australis_ Brongn. previously recorded from New
South Wales. Feistmantel[566] subsequently described a few other Indian
specimens, but did not materially add to our knowledge of the genus.
Bunbury’s specimens were obtained from Bharatwádá in Nagpur, in beds
belonging to the Damuda series of the Lower Gondwana rocks, usually
regarded as of about the same age as the Permian rocks of Europe.
_Phyllotheca indica_ is represented by broken and imperfect fragments
of leaf-bearing stems. The species is thus diagnosed by Bunbury:—“Stem
branched, furrowed; sheaths lax, somewhat bell-shaped, distinctly
striated; leaves narrow linear, with a strong and distinct midrib,
widely spreading and often recurved, nearly twice as long as the
sheaths.” An examination of the specimens in the Museum of the
Geological Society of London, on which this account was based, has led
me to the opinion that it is practically impossible to distinguish
the Indian examples from _P. australis_ described by Brongniart[567]
from New South Wales. The few specimens of the latter species which
I have had an opportunity of examining bear out this view. In the
smaller branches the axis of _P. indica_ is divided into rather short
internodes on which the ridges and grooves are faintly marked. In
the larger stems the ridges and grooves are much more prominent, and
continuous in direction from one internode to the next; a few branches
are given off from the nodes of some of the specimens. The leaves are
not very well preserved; they consist of a narrow collar-like basal
sheath divided up into numerous, long and narrow segments, which are
several times as long as the breadth of the sheath, and not merely
twice as long as Bunbury described them. Each leaf-sheath has the form
of a very shallow cup-like rim clasping the stem at a node, with long
free spreading segments which are often bent back in their distal
region. The general habit of the leafy branches appears to be identical
with that of _P. australis_ as figured by McCoy.
Prof. Zeiller informs me that in the type-specimen on which Brongniart
founded the species, _P. australis_, the sheath appears to be closely
applied to the stem with a verticil of narrow spreading segments
radiating from its margin. It may be, therefore, that in the Australian
form there was not such an open and cup-like sheath as in _P. indica_;
but it would be difficult, without better material before us, to feel
confidence in any well marked specific distinctions between the Indian
and Australian Phyllothecas.
On the broader stems, such as that of fig. 67, we have clearly marked
narrow grooves and broader and slightly convex ridges, which present
an appearance identical with that of some Calamitean stems. In the
specimen figured by Bunbury[568] in his Pl. X, fig. 6, there is a
circular depression on the line of the node which represents the
impression of the basal end of a branch; on the edges of the node
there are indications of two other lateral branches. The nature of
this stem-cast points umnistakeably to a woody stem like that of
_Calamites_. The precise meaning of the ridges and grooves on the cast
is described in the Chapter dealing with Calamitean plants.
[Sidenote: CALAMOCLADUS.]
Grand’Eury[569] in his monograph on the coal-basin of Gard, has
recently described under the name of _Calamocladus frondosus_ what he
believes to be the leaf-bearing axes of a Calamitean plant. The thicker
branches are almost exactly identical in appearance with the broader
specimens of _Phyllotheca_. The finer branches of _Calamocladus_
bear cup-like leaf-sheaths which are divided into long and narrow
recurved segments (fig. 67, B), precisely as in _Phyllotheca_. These
comparisons lead one to the opinion that the _Phyllotheca_ of Australia
and India may be a close ally of the Permo-Carboniferous Calamitean
plants. The form of the leaf-whorls of _Annularia_ (Calamarian
leaf-bearing branches) and of _Calamocladus_ is of the same type as
in _Phyllotheca_; the character of the medullary casts is also the
same. The nature of the fertile shoot of _Phyllotheca_ described by
Schmalhausen from Siberia, with its alternating whorls of sterile and
fertile leaves, is another point of agreement between this genus and
Calamitean plants. An Equisetaceous species has been described from
the Newcastle Coal-Measures of Australia by Etheridge[570] in which
there are two forms of leaves, some of which closely resemble those of
_Phyllotheca indica_, while others are compared with the sterile bracts
of _Cingularia_, a Calamitean genus instituted by Weiss[571].
When we turn to other recorded forms of _Phyllotheca_ many of
them appear on examination to have been placed in this genus on
unsatisfactory grounds. Heer figures several stem fragments from the
Jurassic rocks of Siberia as _P. Sibirica_ Heer[572], and it was the
resemblance between this form and the English _Equisetites lateralis_
which led to the substitution of _Phyllotheca_ for _Equisetites_ in
the latter species. Without examining Heer’s material it is impossible
to criticise his conclusions with any completeness, but several of
his specimens, appear to possess leaf-sheaths more like those of
_Equisetum_ than of _Phyllotheca_.
The frequent occurrence of isolated diaphragms and the comparatively
long acuminate teeth of the leaf-sheath afford obvious points of
resemblance to _Equisetites lateralis_. Some of the examples figured
by Heer appear to be stem fragments, with numerous long and narrow
filiform leaves different in appearance from those of other specimens
which he figures. It may be that some of the less distinct pieces
of stems are badly torn specimens in which the internodes have been
divided into filiform threads. Heer also figures a fertile axis
associated with the sterile stems, and this does not, as Heer admits,
show the alternating sterile bracts such as Schmalhausen has described.
So far as it is possible to judge from an examination of Heer’s figures
and a few specimens from Siberia in the British Museum—and this is by
no means a safe basis on which to found definite opinions—there appears
to be little evidence in favour of separating the fossils described
as _Phyllotheca Sibirica_ from _Equisetites_. This Siberian form may
indeed be specifically identical with _Equisetites lateralis_ Phill.
Various species of _Phyllotheca_ have been described from Jurassic and
Upper Palaeozoic rocks in Australia. Some of these possess cup-like
leaf-sheaths, and in the case of the thicker specimens they show
continuous ridges and grooves on the internodes, as well as a habit
of branching similar to that in some of the Italian Phyllothecas. In
some of the stems it is however difficult to recognise any characters
which justify the use of the term _Phyllotheca_. A fragment figured by
Tenison-Woods[573] as a new species of _Phyllotheca_, _P. carnosa_,
from Ipswich, Queensland, affords an example of the worthless material
on which species have not infrequently been founded. The author of
the species describes his single specimen as a “faint impression”;
the figure accompanying his description suggests a fragment of some
coniferous branch, as Feistmantel has pointed out in his monograph on
Australian plants.
It is important that a thorough comparative examination should be
made of the various fossil Phyllothecas with a view to determine
their scientific value, and to discover how far the separation of
_Phyllotheca_ and _Equisetites_ is legitimate in each case. There is
too often a tendency to allow geographical distribution to decide the
adoption of a particular generic name, and this seems to have been
especially the case as regards several Mesozoic and Palaeozoic Southern
Hemisphere plants.
The geological and geographical range of _Phyllotheca_ is a question of
considerable interest, but as already pointed out it is desirable to
carefully examine the various records of the genus before attempting
to generalise as to the range of the species. _Phyllotheca_ is often
spoken of as a characteristic member of the _Glossopteris_ Flora of the
Southern Hemisphere, and its geological age is usually considered to be
Mesozoic rather than Palaeozoic.
C. _Schizoneura._
The plants included under this genus were originally designated
by Brongniart[574] _Convallarites_ and classed as Monocotyledons.
Some years later Schimper and Mougeot[575] had the opportunity of
examining more perfect material from the Bunter beds of the Vosges,
and proposed the new name _Schizoneura_ in place of Brongniart’s term,
on the grounds that the specimens were in all probability portions of
Equisetaceous stems, and not Monocotyledons. Our knowledge of this
genus is very limited, but the characteristics are on the whole better
defined than in the case of _Phyllotheca_. The following diagnosis
illustrates the chief features of _Schizoneura_.
Hollow stems with nodes and internodes as in _Equisetum_; the surface
of the internodes is traversed by regular ridges and grooves, which
are continuous and not alternate in their course from one internode to
the next. The leaf-sheaths are large and consist of several coherent
segments; the sheaths are usually split into two or more elongate ovate
lobes, and each lobe contains more than one vein. Fertile shoots are
unknown.
Two of the best known and most satisfactory species are _Schizoneura
gondwanensis_ Feist. and _S. paradoxa_ Schimp. and Moug.
_Schizoneura gondwanensis_ Feist. Fig. 69, A and B.
This species is represented by numerous specimens from the Lower
Gondwana rocks of India[576]; it is characterised by narrow articulated
stems which bear large leaf-sheaths at the nodes. The sheaths may have
the form of two large and spreading elongate-oval lobes, each of which
is traversed by several veins (fig. 69, B), or the lobes may be further
dissected into long linear single-veined segments, as in fig. 69, A. It
is supposed that in the young condition each node bears a leaf-sheath
consisting of laterally coherent segments which, as development
proceeds, split into two or more lobes. Feistmantel records this
species from the Talchir, Damuda and Panchet divisions of the Lower
Gondwana series of India; these divisions are regarded as equivalent to
the Permo-Carboniferous and Triassic rocks of Europe. The two specimens
shown in fig. 69 are from the Lower Gondwana rocks of the Raniganj
Coal-field, India.
As already pointed out[577], some of the specimens of flat and
broader stems referred by Feistmantel to _Schizoneura_ are identical
in appearance with stems which have been described from India and
elsewhere as species of _Phyllotheca_.
[Illustration: FIG. 69. _Schizoneura gondwanensis_ Feist. (After
Feistmantel; slightly reduced.)]
There are a few specimens of _S. gondwanensis_ in the British Museum,
but the genus is poorly represented in European collections.
A similar plant was described in 1844 by Schimper and Mougeot[578]
from the Bunter rocks of the Vosges as _Schizoneura paradoxa_. This
species bears a very close resemblance to the Indian forms, and indeed
it is difficult to point to any distinction of taxonomic importance.
Feistmantel considers that the European plant has rather fewer
segments in the leaf-sheaths, and that the Indian plant had somewhat
stronger stems. Both of these differences are such as might easily be
found on branches of the same species. It is, however, interesting to
notice the very close resemblance between the Lower Trias European
plant and the somewhat older member of the _Glossopteris_ flora
recorded from India and other regions, which probably once formed
part of that Southern Hemisphere Continent which is known as Gondwana
Land[579].